Human T-cell growth factor (TCGF, interleukin-2) is a 13,000 molecular weight protein secreted into culture media by PHA-stimulated lymphocytes. This lymphokine has a selective proliferative effect on certain malignant T-cells and lectin or antigen-activated normal T-cells, but no effect on resting T-lymphocytes. In order to gain insight into the mechanism of its selective mitogenic effect, human TCGF, purified by ion exhange chromatography, gel filtration, and preparative SDS-polyacrylamide gel electrophoresis, and labelled with I125 or activated biotin, will be employed to determine the spectrum of cells with membrane receptors for TCGF. Binding studies with labelled TCGF will involve a survey of normal and malignant hematopoietic cells, with an emphasis on cutaneous T-cell lymphomas and T-cell leukemias. The presence of TCGF receptors, inferred from the ability of cells to bind labelled growth factor, will be correlated with the proliferative response induced by the factor, assayed by 3H-thymidine incorporation, and with cell surface markers. Biotin-TCGF will be used with fluorescein-Avidin in the Fluorescence Activated Cell Sorter to correlate the presence of TCGF receptors with cell size, DNA content, and the presence of other surface markers such as Ia antigens, peanut agglutinin receptors, and surface antigens recognized by monoclonal antibodies directed against specific T-cell subsets. The blocking of the binding of biotin-labelled Interleukin-2 to activated T-cells will be used as a screening assay for the production of monoclonal antibodies against the Interleukin-2 receptor.